1) Field of the Invention
The present invention relates to an optical fiber, a Raman amplifier using the optical fiber, and an optical communication system using the optical fiber.
2) Description of the Related Art
A Raman amplifier is characterized in that the optical signal transmission distance is extended and noise in optical fiber transmission is reduced. As the Raman amplifier, a distributed Raman amplifier and a discrete Raman amplifier are known.
The distributed Raman amplifier includes of first and second pump light sources, optical multiplexers and an optical fiber. This distributed Raman amplifier is also applied to the embodiment of the present invention.
Meanwhile, the discrete Raman amplifier is an optical amplifier constituted so that an amplified light is inputted to an optical fiber in an enclosed device, for example, accommodates the coiled optical fiber and is installed in a relay station, such as a dispersion compensating module. A dispersion compensating module is typical of the assembled optical fiber.
If the optical fiber is made of silica glass, a maximum gain peak of Raman amplification appears at an optical frequency lower by 13 terahertz (in other words, about 100 nanometers) than that of a pump light. In the optical communication system in a 1.5 micrometers band, for example, the pump light should be set to have a wavelength of 1480 nanometers so that a signal light at a wavelength of 1580 nanometers can attain a maximum Raman gain.
In a wavelength division multiplexing (hereinafter, “WDM”) system, a pump light at a short wavelength amplifies a signal light at a short wavelength and the pump light at a long wavelength amplifies the signal light at a long wavelength.
In the optical communication system, if a zero-dispersion wavelength of the optical fiber is between the wavelength of the signal light and the wavelength of the pump light, four-wave mixing (hereinafter, “FWM”) due to the signal light and the pump light, which is a nonlinear phenomenon, occurs nearly at the wavelength of the signal light and deteriorates transmission characteristics of the system.
Meanwhile, if the FWM generating efficiency increases, the pump light exploited by the FWM increases and thus the signal light cannot attain a high Raman gain.
Countermeasures against such a disadvantage using the optical fiber having a zero dispersion wavelength between the wavelengths of the signal light and the pump light is to lower the intensity of the pump light. However, this is not a good solution since it causes a decrease in Raman amplification gain.
If the zero-dispersion wavelength of the optical fiber is smaller than the wavelength of the pump light, the FWM due to the signal light and the pump light can be suppressed without decreasing the Raman amplification gain.
An Erbium-doped fiber amplifier (hereinafter, “EDFA”) with an Erbium-doped optical fiber allows an optical signal transmission in C-band (e.g. the wavelength band of 1530 to 1565 nanometers) in a Dense Wavelength Division Multiplexing (hereinafter, “DWDM”) system. Recently, optical transmission using the signals in L-band (e.g. the wavelength band of 1565 to 1625 nanometers) is actively introduced.
As explained, the Raman amplifier communication system requires the pump light with a wavelength shorter by about 100 nanometers than the minimum wavelength of the signal light. This indicates that the optical fiber for Raman amplification in both C-band and L-band should not have a zero-dispersion wavelength between 1430 nanometers, shorter by 100 nanometers than 1530 nanometers, and 1625 nanometers.